Numerous forms of sterile state indication means or sterile state indicators for sterilization containers are known in prior art. Such indicators serve to signalize the staff if a sterilization container has already been sterilized or not. Said sterile state indicators respond to heat and/or vapor and change their color during the sterilization process. Subsequent to the sterilization process, the staff is able to determine on the basis of the color of the indicator spot that the content of the container is sterile. The indicator spots are attached either on labeling cardboard or on a container seal. If the indicator spot is provided on a labeling cardboard, however, there is the risk that such a labeling cardboard, after the use of a container and its content, is not removed and replaced by a new labeling cardboard; instead, the labeling cardboard may remain on the container and in this way erroneously indicate that the container together with its content has been freshly sterilized.
If the indicator spot is on a container seal which has to be destroyed for opening the container, it cannot happen by accident that the old indicator spot remains on the container. With some container seals, however, it is difficult to clearly see if the seal has already been destroyed. In this case, an old seal might be attached to a container and thus indicate in erroneous fashion that the content of the container is sterile. In both cases, however, the indicator has to be replaced after every use, involving an extra working step and the stockage of the indicators or the seals along with indicators.
Other sterile state indication systems work with bimetallic springs or shape memory metals which are deformed during the sterilization process mostly against a regular spiral spring and displace a display element in order to lock it in place under pretension of the regular spiral spring. If the container is opened, the locking of the display element is released and it returns to a position which does not indicate the sterile state any longer. However, these systems have the disadvantage that they require a very complex structure which calls for many individual movable parts. Moreover, these systems need a considerable construction volume which reduces the usable volume of the container. This results in considerable manufacturing costs. Components made up of shape memory metals are also very expensive. On the other hand, these systems can be used in reversible fashion and do not have to be replaced before each sterilization process as is the case with the previously described indicator spots.
In the prior art, so-called phase change materials (PCM) are also known. Phase change materials are usually used for latent heat accumulators. An insight into the basics of latent heat accumulators and phase change materials can be found in the paper “Latent heat accumulators—functional principle and fields of use” by Petra Oberpaul, 2002. In said paper, solar energy production, housebuilding or the automotive industry are mentioned as application areas, for example. Further, a distinction is made between three classes of phase change materials, namely eutectic water-salt solutions, organic phase change materials and salt hydrates. Eutectic water-salt solutions are mainly used for cold storage purposes. Paraffins (long-chain hydrocarbons) and sugar alcohols such as erythritol, mannitol and sorbitol belong to the organic phase change materials. In the intended temperature range between room temperature and the sterilization temperature, salt hydrates are the favorite alternatives. Examples for salt hydrates include calcium chloride hexahydrate, sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate, disodium thiosulfate pentahydrate, sodium acetate trihydrate, barium hydroxide octahydrate and a mixture of magnesium hydrate hexahydrate and lithium nitrate.